The idiom "了如指掌" (liǎo rú zhǐ zhǎng) is formed through a vivid metaphorical structure: the first character "了" means "to understand clearly", which is then compared by "如" ("like" or "as if") to the visual clarity and intimate familiarity one has with "指掌", the lines on one's own palm ("指" for fingers and "掌" for palm). Thus, the entire phrase constructs an image of knowing something as thoroughly and effortlessly as one knows the lines on their own hand, culminating in a meaning of having complete and detailed mastery or understanding of a subject. It functions as a predicate in modern usage, often following subjects like 对... ("towards something").